1. Field of the Invention
This invention relates to a fuel control apparatus for an internal combustion engine.
2. Prior Art
A wide variety of fuel control apparatuses have been used for providing optimum air-fuel ratioes. FIG. 7 shows one such prior art fuel control apparatus described in Japanese Patent Preliminary Publication No. 60-212643. A crank angle sensor 7 outputs a reference position pulse for each reference position of crank angle (every 180 deg. for four-cylinder engine and every 120 deg. for six-cylinder engine) and a unit angle pulse for each unit angle (eg. one degree). Thus, the crank angle can be determined by counting the unit angle pulses after the reference position pulse is inputted into a control apparatus 12. Further, the rotational speed of the engine can be determined by measuring the frequency or period of the train of unit pulses.
In FIG. 7, the crank angle sensor 7 is provided in the distributor.
The control apparatus 12 is formed of, for example, CPU, RAM, ROM, and I/O interface. The control apparatus 12 receives an intake-air flow rate signal S1 from an air flow meter 2, a water temperature signal S2 from a water temperature sensor 6, a crank angle signal S3 from the crank angle sensor 7, an exhaust signal S4 from an exhaust sensor 9, and a battery voltage signal and a fully-closed throttle signal (not shown), and calculates a fuel amount to be injected on the basis of these signals to provide an fuel injection signal S5. A fuel injection valve 10 is actuated by the fuel injection signal S5 to supply the engine with a required amount of fuel.
The fuel injection Ti to be injected is calculated by the control apparatus 12 using the following equation. EQU Ti=Tp (1+Ft+KMR/100) .beta.+Ts (001) EQU Tp=KQ/N
where Tp is a basic injection amount, Q is an intake air flow rate, N is a rotational speed of the engine, and K is a constant.
Ft is a correction factor dependent on the temperture of cooling-water of the engine, which is increasingly large with decreasing temperature. KMR is a correction factor when the engine is heavily loaded, and is read through table-look-up from a data table in which sets of data dependent on the basic injection amount Tp (ms) and the rotational speed N (rpm) are stored in advance as shown in FIG. 8. Ts is a correction factor for correcting fluctuation of the voltage which drives the fuel injection valve 10. .beta. is a correction factor dependent on the exhaust signal S4 from the exhaust sensor 9. Through the use of 62 , the air-fuel ratio of the mixture can be feedback-controlled to a predetermined value, for example, a value close to the theoretical air-fuel ratio of 14.6. Where feedback control based on the exhaust siganl S4 is underway, the air-fuel ratio of the mixture is controlled to a constant value, in which case the corrections for the cooling-water and heavy load are meaningless. Thus, the feedback control using the exhaust signal S4 is carried out only when the correction factors Ft and KMR are zero. FIG. 9 illustrates the relation between the various sensors and the respective corrections calculated on the outputs of these sensors. For example, the signal from the air flow meter 2 is used to calculate the basic injection amount, the heavy load correction, and an injection amount when the engine is just started.
In the prior art fuel control apparatus described above, the intake air flow rate Q is measured by the air flow meter 2, and is then divided by the rotational speed N to obtain the basic injection Q. Thus the air flow meter 2 plays a fundamental role in the fuel control apparatus. The prior art apparatus suffers from the following drawbacks.
(1) An air flow meter is normally installed upstream of a surge tank. Therefore, during transient period in which the throttle opening changes abruptly, it measures not only the intake-air flow rate of the air flowing into the engine but also variations of the amount of air trapped in the inlet pipe (i.e., amount of air flowing into the inlet pipe), causing a difficulty in measuring an actual amount of air flowing into the engine and therefore disturbing the control of the air-fuel ratio.
(2) A large air flow meter is required, which is not preferable from a point of view of space factor.
(3) The output of the air flow meter is directly used to determine the fuel injection. This requires an accurate air flow meter.
Japanese Patent Preliminary Publication No. 59-221433 discloses a procedure for measuring the pressure in a combustion chamber to calculate an amount of air charged into the combustion chamber. As is apparent from FIG. 11, the air charge amount Ga is in a linear relation with the pressure difference .DELTA.P within the cylinder, where .DELTA.P is the pressure difference within the cylinder between the bottom dead center (BDC) and 40 deg. before the top dead center (BTDC 40 deg.) as shown in FIG. 10. The air charge amount is calculated on the basis of .DELTA.P by using this relation. However, this procedure suffers from a drawback that the measurement accuracy is directly dependent on the gain of the sensor since a change in gain causes a change in the pressure difference .DELTA.P for the same air charge amount.